In a power conversion system, such as a variable-speed, constant-frequency (VSCF) power generating system, a generator, typically a brushless, three-phase synchronous generator, is operated in a generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The prime mover may be, for example, a gas turbine engine of an aircraft. The variable-frequency AC power produced by the generator is rectified and provided as a DC signal over a DC link to an inverter. The inverter inverts the DC signal on the DC link into constant-frequency AC power for supply over a load bus to one or more AC loads.
As is known, a generator can also be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring the prime mover up to self-sustaining speed. For example, when operated in a starting mode, the generator can be used to start a gas turbine engine of an aircraft.
One type of a brushless, synchronous generator, which can be operated in both a generating mode and a starting mode, includes a permanent magnet generator (PMG), an exciter, and a main generator all mounted on a common shaft. When such a generator is used in a starting mode, it is known to provide AC power at a controlled voltage and frequency to the armature windings of the main generator and to provide AC field current to the main generator field windings by way of the exciter in order to operate the generator as a motor to produce starting motive power. Respective inverters normally supply the AC power to the main generator armature windings and the AC field current to the exciter.
In order to properly accelerate and control the generator, and thereby the prime mover, during operation of the generator in its starting mode, it is necessary to properly commutate or switch the currents supplied by the inverter to the armature windings of the main generator in synchronism with the position of the shaft of the generator. In the past, synchronization was achieved by using a position sensor such as a resolver, a synchro, an optical encoder, or hall effect devices. For example, in Lafuze, U.S. Pat. No. 3,902,073, three hall effect sensors are mounted in an air gap of a permanent magnet generator so that they are 120.degree. (electrical degrees) apart with respect to the permanent magnet rotor pole pairs of the permanent magnet generator. As the rotor of the permanent magnet generator rotates, the voltage across each hall effect sensor varies from zero to a maximum as a function of rotor position, thereby generating three generally trapezoidal voltages spaced apart in phase by 120.degree.. Thus, the outputs from the hall effect sensors are representative of the position of the permanent magnet generator rotor. The output signals from the hall effect sensors are used to control switching elements in the inverter which supplies the AC power to the armature windings of the main generator.
The use of such position sensors entails considerable expense in the position sensor itself and its associated electronics, and further results in extra wires and extra assembly steps to install the position sensing apparatus. Also, operational parameters often limit the accuracy of such position sensors.
In view of the foregoing difficulties, other approaches have been taken in an effort to detect rotor position without the need for such position sensors. In the case of a brushless DC motor control, a back EMF approach has been used to detect rotor position. The back EMF of the motor is defined by the following equation: EQU E.sub.emf =K.omega.Sin.alpha.
where K is a constant, .omega. is the angular speed of the motor, and .alpha. is the electrical phase angle of the rotor. From the foregoing equation, it can be seen that, since the angular speed .omega. of the motor is known or can be easily detected and since the back EMF of the motor can be detected, the rotor electrical phase angle .alpha. can be determined. The electrical phase angle .alpha. of the rotor is equivalent to rotor position and can be used in the proper commutation of the currents supplied to the armature windings of the motor.
The back EMF voltage can be detected using either of two methods, referred to as the direct method and the indirect method. The direct method can be used to directly measure phase of the back EMF voltage, but only when the phase winding is not energized by the inverter connected thereto and the winding is not short circuited either by closed switches in the inverter or by conducting flyback diodes in the inverter. Such conditions can be realized when a 120.degree. commutation algorithm is utilized. In this case, a voltage reading is taken after a short delay following switching off of the phase winding to ensure complete current decay by the free-wheeling diodes. This direct method is described in a paper entitled "Microcomputer Control for Sensorless Brushless Motor" by E. Iizuka et al., IEEE Transactions on Industry Application, Vol. IA-21, No. 4, May/June 1985.
The indirect method is based on estimating the back EMF from the motor terminal voltage and phase cur-rents. This method is suitable for both 120.degree. and 180.degree. commutation algorithms. One technique that uses this method is described in a paper entitled "Position--and--Velocity Sensorless Control for Brushless DC Motor Using an Adaptive Sliding Mode Observer" by Furuhashi et al., IEEE Transactions on Industrial Electronics, Vol. 39, No. 2, April 1992.
Because the back EMF voltage of a motor is zero at standstill and the signal to noise ratio is very small at lower speeds, reliable determination of rotor position at low rotor speeds is difficult.
A method of using a permanent magnet generator as a position sensor for operating a generator in a starting mode is described in Stacey, U.S. Pat. No. 5,140,245. A standard brushless generator is equipped with a permanent magnet generator which is used as an emergency electric power source and as a source of control power when the brushless generator is operated normally as a generator. The permanent magnet generator develops a multiphase output which is supplied to a high resolution phase-locked loop having a binary counter which develops an output signal representing shaft position. In order to avoid ambiguous position readings, however, this method is limited to the situation where the number of permanent magnet generator rotor poles is equal to, or less than, the number of poles on the main generator rotor.